1. Field of the Invention
This invention relates to a method and apparatus by which the rate of accumulation of polymeric or other material such as but not limited to diatemaceous earth, wax, and calcium carbonate on the lip of an extrusion die, in particular, a polymeric film extrusion die is reduced.
2. Description of the Prior Art
Extrusion of polymers through a die to form shaped extrudates is a well known process, and is extensively used commercially. Items that can be made by polymer extrusion include films, fibers, sheets, strands, and shaped profiles. The extrudates are usually essentially continuous and are often cut into usable lengths.
The extrusion process is one in which the molten polymer is forced through a shaped orifice, and the extrudate is then cooled, usually until solid. The cross section of the extrudate (perpendicular to the flow direction) usually has the same shape as the orifice. Due to the importance of extrusion commercially, improved dies are constantly being sought so that the process runs more cheaply and/or with less off-quality production. A particular problem in this process is the accumulation of small portions of molten polymer other material such as but not limited to diatemaceous earth, wax, and calcium carbonate around the die orifice which can cause problems of product defects and downtime. In the case of film and sheeting, the material collecting along the length, and adjacent to, the die orifice can cause a defect in the surface of the cast sheet or film known as a die line. Such a die line results from a sharply irregular wetting contact at the corner of the die orifice where the extruded sheet breaks contact with the metal of the die orifice. The build-up of foreign materials adjacent to the die orifice provides sites for such irregular wetting contact. This foreign material can take several forms and can be caused by many different phenomena. In the case of flat sheeting, such a build-up of foreign material at the two ends of the orifice can cause ragged and non-uniform edges on the extruded sheet.
While there are many recognized sources for the foreign materials that cause die lines, and/or ragged and unsharp edges and/or other defects in the extruded sheet, one of the most difficult to control is the slow accumulation of material build-up directly adjacent to the orifice.
One attempt at solving this problem is taught by U.S. Pat. No. 3,502,757 to Spencer. There, it is indicated that certain polymeric thermoplastics including polyamides, polycarbonates, and polyesters, and more particularly, polyethylene terephthalate, give up various vapors and low molecular constituents in a xe2x80x9cflashxe2x80x9d type of process at the instant that the material which is extruded under pressure is exposed or reduced to atmospheric pressure as it leaves the die orifice. The presence of high concentrations of these vaporous contaminants in the area directly adjacent to the extruding plastic is indicated to be the cause of the material build-up. To resolve this problem, the ""757 patent teaches to replace the contaminated air in the immediate vicinity of the extrusion orifice with clean air by directing an air stream against one or both sides of the extruded sheet.
An alternate approach is taught by U.S. Pat. No. 5,458,836 wherein the focus of the invention disclosed therein is to modify the orifice shape. In particular, the ""836 patent teaches a die having flared orifices for the extrusion of polymers having a plurality of zones of given lengths defined by corresponding surfaces and the relative dimensions of the zone lengths and surface configurations.
U.S. Pat. Nos. 3,734,984 and 3,847,516, both to Hoffman, teach extrusion of a molten polymeric plastic under pressure through an orifice while impinging a blast of clean hot air or other gas against the extruded product at the instant it leaves the orifice and introducing into the gas, as by steam injection or by water sprays, water vapor so as to maintain its humidity at a level corresponding to a dew point of at least 35 F., and preferably 50-80 F. U.S. Pat. No. 5,417,907 to Ohhata et al., teaches a die plate for use in the extrusion of plastics which is capable of preventing or substantially reducing the formation of resin masses at the die face of the die plate by providing an extrusion orifice having an increased diameter in the vicinity of the die face.
Yet other means for reducing the accumulation of material on the edge or lip of a film extrusion die include the addition of additives to the polymer.
In spite of the advances that have been made, the problem continues to exist.
Accordingly, it is one object of this invention to provide a method and apparatus for reducing the rate of accumulation of material on the edge or lip of a polymer extrusion die.
This and other objects of this invention are addressed by a method comprising the steps of machining the polymer exit edge of a polymer extrusion die to a thickness of less than about 0.005 inches, passing a molten polymer through the extrusion die, forming a polymeric extrudate, and blowing a heated gaseous fluid onto the polymer exit edge. The shape of the die lip is such that the 0.005 inch edge is adjacent to the polymer at the exit of the die. Due to the machined edge at the polymer exit, there is essentially no surface remaining on which material can accumulate.
The apparatus for reducing the rate accumulation of material on the lip of a polymer extrusion die in accordance with one embodiment of this invention comprises a polymer extrusion die comprising a die body having a molten polymer inlet and a molten polymer exit and forming a molten polymer conduit providing a polymer communication between the molten polymer inlet and the molten polymer exit. The die body further comprises a peripheral edge at a periphery of the molten polymer exit, which peripheral edge has a thickness of less than about 0.005 inches. Edge thicknesses above about 0.005 inches normally do not provide the desired result. A molten polymer exit heated air supply adapted to provide heated air to the polymeric extrudate is disposed proximate the molten polymer exit.